An active magnetic bearing which may be a radial bearing or an axial (or thrust) bearing comprises a rotor, a stator fitted with electromagnet windings, at least one sensor for sensing the radial or axial position of the rotor and servo-control circuits for maintaining the rotor in equilibrium without contact with the stator, the currents carried by the electromagnets of the stator being servo-controlled on the basis of signals delivered by the at least one sensor.
FIG. 9 schematically shows an example of a known radial magnetic bearing 200 comprising a stator with a stack of ferromagnetic laminations 211 and electromagnet windings 212 and a rotor fitted with a second stack of laminations 213 mounted on a shaft 220. A radial position detector 201 of the inductive type comprises a stator with a stack of ferromagnetic laminations 231 and electromagnet windings 232 and a rotor fitted with a second stack of laminations 233 mounted on the shaft 220. An axial position detector 202 of the inductive type comprises a stator with stacks of ferromagnetic laminations 251, 251′ and electromagnet windings 252, 252′ and a rotor fitted with second stacks of laminations 253, 253′ mounted on the shaft 220. The servo-control circuits are not illustrated in FIG. 9. Active magnetic bearings may be configured in various manners. In particular, as disclosed in U.S. Pat. No. 6,849,979 B2, radial and/or axial position sensors may be combined and/or integrated in a bearing. Moreover the ferromagnetic laminations may also be replaced by solid parts of a magnetic material.
When the position sensors are of the inductive type, the rings of rotor laminations such as the stacks of ferromagnetic laminations 233, 253 and 253′ in FIG. 9 and their support are called a rotor sensor target.
FIG. 10 illustrates an example of a known rotor sensor target. Target materials 230, 240 are mounted on a shaft 220 which is typically made of carbon steel and are made integral thereto.
Generally speaking to make an axial or an axial-radial sensor on the rotor of a magnetic bearing system, two materials are needed, i.e. a first non-magnetic material and a second magnetic material, which may be made of laminations or of a solid part. The axial displacement of the rotor is measured at the border between the magnetic and the non-magnetic materials. These materials need to have sufficient strength to withstand high speeds and different temperatures without losing contact to the main shaft or breaking.
In the known embodiment illustrated in FIG. 10, Inconel 718 may be used as a non-magnetic material constituting a main target component 240 in combination with laminations 230 of magnetic material. The main target component made of non-magnetic material may comprise a ring 241 having a height H1 which is mounted on the shaft 220 of carbon steel having a height H2.The laminations 230 of ferromagnetic material are mounted on the ring 241 and are maintained between an additional ring 242 of non-magnetic material and a projection 243 of the ring 241.
The advantage given by the Inconel as a non-magnetic material is its very high mechanical resistance together with a coefficient of thermal expansion very close to the steel coefficient of thermal expansion. The addition of these two properties allows the sensor to be used in a wide range of temperatures and speeds. However there is a disadvantage of a very high cost due to the Inconel price.
As an alternative material to Inconel a design with a high resistance stainless steel (Z6NiCrTiMoVB25-15-2) or High resistance brass (CuNi2Si) has been used for the non-magnetic material 241, 242 to reduce the cost compared to Inconel with little restrictions on temperature and speed but a lower cost. The laminations of magnetic material 230 are usually made of FeSi.
As shown in FIG. 10, with the conventional shape of the ring 241 with a projection 243 and of the additional ring 242 all made of non-magnetic material such as Inconel, high resistance stainless steel or high resistance brass, the height H1 of the material under the magnetic iron laminations 230 interposed between the ring 242 and the projection 243 and the addition of the height H2 (i.e. the radius) of the shaft 220 under the magnetic portion 230 of the sensor target gives a very stiff assembly which may be detrimental to the magnetic portion made of laminations if some cheaper non-magnetic materials are used for the ring 241 with projection 243 and the ring 242.